Researchers have uncovered unexpectedly hungry little black hole monsters. Using the Subaru Telescope, they've found evidence of ultra-luminous X-ray sources (ULXs) which provide evidence of small black holes that are consuming matter at a phenomenal rate.

X-ray observations of nearby galaxies have revealed that these exceptionally luminous sources at off-nuclear positions that radiate about million times higher power than the sun. The origins of ULXs have been a subject of heated debate for a long time, and the basic idea is that a ULX is a close binary system consisting of a black hole and a star. As matter from the star falls onto the black hole, an accretion disk forms around the black hole. One the gravitational energy of the material is released, the innermost part of the disk is heated up to a temperature higher than 10 million degrees, which causes it to emit strong X-rays.

Yet what exactly is the mass of the black hole in these bright objects? There are two different black hole scenarios proposed; either they contain very "big" black holes that could be a thousand times more massive than the sun, or they could be relatively small black holes with masses no more than a hundred times that of our sun.

In order to find out which was the case, the researchers examined high-quality data from several objects. In all of the optical spectra of the ULXs observed, the researchers found a common theme: a broad emission line from helium ions, which indicated the presence of gas heated to several tens of thousands of degrees in the system. This suggested that the gas must be accelerated outward as a wind from either the disk or the companion star. In the end, the researchers concluded that huge amounts of gas must be rapidly falling into "little monster" black holes in each of the ULXs observed.

The findings reveal a bit more about these black holes and show that instead of giants, they're actually voracious, small "monsters."